Optical ranging and velocimetry systems became viable with the advent of the laser. However, coherent detection systems, i.e., systems which utilize laser beams have heretofore involved some variation of unequal path interferometry. While such systems have proven successful, they are burdened with complex precision optics, large size and sensitive alignment. While laser diodes have lessened the cost and complexity of such systems, many of the problems, e.g. complex optics and sensitive alignment problems persist.
However, it has been discovered that the problems of complex optics and alignment sensitivity may be overcome by utilizing the laser light backscattered from a target to directly modulate the source laser. In other words, the scattered light is focussed back into the laser gain medium. The doppler shifted backscattered light produces a beat frequency with the source light. The beat frequency is related to target velocity by the doppler effect and can be related to target distance by frequency modulation of the source light as in the chirp radar technique.
The above system of mixing the backscattered light with the laser source light referred to sometimes as the "self-mixing" method requires only one common lens and is self-aligning.
The present invention makes use of the above discussed techniques in carrying out its objectives.